Process for pickling stainless steel in the absence of nitric acid and in the presence of chloride ions

a technology of stainless steel and chloride ions, which is applied in the direction of detergent compounding agents, lighting and heating apparatus, cleaning using liquids, etc., can solve the problems of excessive unevenness and corrosion, surface, and greater difficulty,

Inactive Publication Date: 2003-04-29
HENKEL KGAA +1
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Problems solved by technology

In industrial practice, the situations presenting greater difficulty are found in the treatment of duplex steels, austenitic steels with high chrome content, and chrome-nickel alloys.
Considering that in the pickling process, above all in wire-rod and tube pickling, the pickl

Method used

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  • Process for pickling stainless steel in the absence of nitric acid and in the presence of chloride ions
  • Process for pickling stainless steel in the absence of nitric acid and in the presence of chloride ions
  • Process for pickling stainless steel in the absence of nitric acid and in the presence of chloride ions

Examples

Experimental program
Comparison scheme
Effect test

example 2

Specimens of austenitic stainless-steel wire rod of the type for the production of electrodes in compliance with the DIN 1.4430 Standard, which had a diameter of 5.5 mm and a length of approximately 10 cm and had been hot-rolled, were subjected to pickling treatment in solutions with different chemical compositions in the absence of chloride ions and in the presence of 3.0 g / l of chloride ions introduced as sodium chloride with the purpose of not altering the acidity of the solution. For each test, a quantity of pickling solution of 1000 ml was used.

In ordinary industrial practice, for this material to be chemically pickled within an industrially acceptable time, it is subjected to pre-treatment in an oxidizing or reducing fused-salt bath.

The specimens of material in the test in question were not subjected to any pre-treatment stage in order to compare them to specimens of the same material previously treated in oxidizing fused salts and pickled in a solution of the same composition...

example 3

Pipes made of austenitic steel of the NiCr23Fe series (60% Ni, 23% Cr--specimen {character pullout}) and of the super austenitic-duplex series, such as (22% Cr, 5% Ni, 3% Mo--specimen {character pullout}) or else (25% Cr, 7,5% Ni, 4% Mo--specimen {character pullout}) can be chemically pickled only with very long treatment times and with very high acid concentrations.

In the case of the NiCr23Fe series, pickling is moreover possible only after pre-treatment in an oxidizing solution, such as a molten-salt solution or alkaline-permanganate solution.

The treatment of these steels in solutions containing chloride ions in the range of the present invention enabled pickling to be carried out in much shorter times than using processes without chloride in the three cases considered.

example 4

Specimens of hot-rolled and sand-blasted strip made of stainless steel of the AISI 304 series and having a thickness of 4.8 mm underwent pickling in solutions containing HF, H.sub.2 SO.sub.4, Fe.sup.3+, and Fe.sup.2+, both in the absence and in the presence of chloride ions.

The total stay time of this material on the production line was approximately 3 minutes, during which the material passed through two pickling tanks and underwent, both after the first tank and at end of cycle, an operation of washing+mechanical brushing.

At the end of the process, the mean weight loss for this material was in the 13-18 g / m.sup.2 range.

The laboratory tests were conducted on a liter of solution in the presence of agitation by blowing in air, but in the absence of any mechanical action. The result must therefore be interpreted for comparison with the original solution.

The result was assessed by measuring weight loss and surface appearance of the test specimen every 60 seconds of pickling and at the ...

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Abstract

Process for pickling stainless steel carried out at a temperature of between 20° and 70° C., with the use of a pickling solution containing the following basic ingredients:H2SO4 (free acid): 50 to 200 g/lHF (free acid): 0 to 60 g/lF- anion (total): 5 to 150 g/lSO42- anion (total): 50 to 350 g/lTotal free acidity (H2SO4+HF): between 1 and 7 g. equiv./l where by "free acid" is meant the acid that does not constitute the anion bound in the form of salts or complexes with the metal cations present in the solution; and, in addition:Fe3+ in a quantity of at least 15 g/lchloride anion in a quantity of between 0.1 and 10 g/l,into which solution is fed, during the pickling process, an oxidant that is able to oxidize Fe2+ to Fe3+ in order to maintain the redox potential of the solution at a value of between +230 and +800 mV.

Description

TECHNICAL PROBLEMIn the course of processes of manufacture of steel products that undergo hot-rolling operations or of intermediate products that undergo thermal treatment, such as annealing, it has long been known that the material becomes coated with a more or less thick layer of products of oxidation. It is therefore necessary, considering the need to obtain a degree of surface finish of the finished product that is smooth and shiny, to removing entirely such oxide layers. This is performed through the well-known processes of pickling, for which mixtures of inorganic mineral acids, such as hydrochloric, sulphuric, nitric, and hydrofluoric acids, either alone or in various ratios between them, are generally used.In the field of stainless steels, on the basis of the knowledge of the industrial processes today in use, the most commonly employed pickling operation involves the use of a mixture of nitric acid and hydrofluoric acid, the reciprocal concentrations of which in the mixture...

Claims

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Application Information

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IPC IPC(8): C23G1/08C23C22/05C23C22/50C23C22/34
CPCC23C22/34C23G1/086C23C22/50Y10T29/45
Inventor NEGRI, DARIOGIORDANI, PAOLO BRUNO
Owner HENKEL KGAA
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